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Full text of "The history and development of [the] WRC transmitter / by Wilbur M. Herbert"

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'(Vilbur },?. .'ierbert 

Hovember 17, 1939 


The new ITOC transmitter vms built and placed into operati; n 
after definite evidence had been obtained showin^^ that a new set up 
was necessary. The selection cf a site was an important problem to sol're, 
and the choice of the most desirable pov.'er to be considered. Con- 
struction and erection of antenna and ground system was next considered. 
The transmitter proper is a complicated piece of equipment and dis- 
cussion YFBB limited. The acquisition of power, emergency facilities, 
and antenna-transmitter coupling were described. The final consideration 
T^as t^e completion of the transmitter and the results of test performed. 


On March 15, 1938 V/RC silenced forever the old traasmitter 
which had faithfully and continuously served the Nation's Capital for 
fifteen year?:, and replaced it 7/ith a nevr transmitter. Yifashington' s 
newest and most modern transmitter is located approxiinately six miles 
northeast of the capital in Chillum, Maryland. 

liVTIC selected a new transmitter site because the station desired 
to provide better service to the iria.iority of its listeners, by moving 
to a more suitable locaticn. They desired to increase the power and 
found it waF necessary to relocate in order that excessive blanketing did 
not occur. 


Considering the magnitude of the investment end the possible 
cost and inconvenience of moving to a more suitable site at son.e future 
time t e selection of a new location became of great importance. 
Therefore, the engineers looked for a site which would stand up under 
the follo'.ving requirements: (l) The site must be so situated that 
blanketing of an excessive percentage of receivers will not be produced, 
(2) The site should be so located with respect to the service area that 
a s'^itable directive antenna will furnish the desired protection, if 
this protection is necessary, v/ithout undue limitaticn of the service 
'.ree, (?) The s i e should provide #-v maximiim signal foi the greatest 
possible number ci listeners. The city iinmedit:itely adjacent to the 
station should receive not less than approxim-=itel;'' 50 millivolts per 
meter to insure r-ood service under adverse noise and atmospheric dis- 
turbances. (4) The site should preferably be near e'^tablish d power 
and telephone facilities. (,5) The site should be located on terrain 
that permits good efficiency from the antenna system. This means, under 
ideal conditions, level r'srshj'' land. \6) The site should be near a 


main highi'my in order that it oe accessible imder adverse weather 
conditions. (7) There should be available, at reasonable cost, a suf- 
ficient area of land to aoconmiodate the antenna and :^-round s,^ stem, (b) 
The antenna system s-^ould not be an undue hazard to established airlines. 

After consideration of the above req. irements the possi 'le sites 
were sele'-ted, and on the one seemine most premising a test transmitter 
and antenna were placed. This test transmitter is generally a low- 
power portable or semi-portable transmitter, capable of deliverinr about 
100 watts into the anteiina Vfith good frequenc' , stability, and little 
fluctuation of output power. The antenna may be any conveniently erected 
vertical antenna. Usually in order to facilitate loading; and to reduce 
loading losses, it should be rreferabl.^' of cage type. The signals sent 
out from this test outfit were pickec up by an automobile radio, this 
w&% done at nipht. "^he data was compiled from tests, the selection of 
the site being governed by the results. 

On the basis of these tests a site was chosen at Chilluin, 
MarylEind. These tests showed that listeners in the District of Colu;;"bia, 
Virginia, and ' aryland Tvould be provided with a nev/ high in fidelity of 
transmission and absence of noise. • 


Interesting results were obtained wher wash borings yrere 
examined as a guide to foundation design. The area on' which this station 
is built was once a valley, the floor of which was ZC< feet belov^ the exist- 
ing grou- d level, ^'he borings indicated unv;su-lly interesting sell 
strata, not the leart of which was a layer of decayed wood and vet-eta- 
tion. '-^"he wood was Ir a fair state of preservation althoup-h having been 
buried for thousands of years. From a depth of only 12 feet in one of 





the guy anchor excavations, decayed wood •va fovnd which was in an 
advanced stap;e of turninr, to coal. 

Altliough trie soil strata was unusual, arid swampy terrain of the 
type encountered ma;;es possible efficient radio transmission, it provides 
conspicuously poor foundation conditi.iis for antenna?-, and buildings. As 
a result, the guy anchors, toi-.^er, and buildirj^s are supported on 30 foot 
wood fer.d concrete piles, 'ihe bidlding, antenna pier and guy anchors are, 
in addition, built several feet above the majcimum recorded flood vmter 
level. Since the station has nothing to fear from floods, f=nd ground 
conditions are improved by them, the station staff are ]:repared to 
accept them with equanimit; if not enthusiasm. 


The new antenna is of uniform cross section from tor to bottom, 
ard is triangular in shape, with faces five feet vride. At the top of 
this slender, but strong structure there is a steel capacity top 50 feet 
in diameter. This so called "high hat" i? t"ne top tuning structure which 
^:BG had pioneered at KOA in 1933, WTF and WHIO in 1934, snd IVMAQ in 1935. 
It has been developed to a high state of effectiveness. 

Although it is usually desirable to erect an antenna to the 
optimum height, restr'ctions are frequently imposed by the United St^^tes 
Department o ' Aeronautics and the Federal Coinmunication Commission, 
which make it necessary to resort to oti er expedients in order to obtain 
satisfactor^■ efficiency. This occured in the case of '"<T?C. The 50 foot 
diameter steel top, used in connection with the authorized height of 
400 feet, made it possible to approach the desired results without 
supplementary insulation and inducta. ce loading at the top of the 


The specifications for the iiVRC vertical radiater or antenna 






-c' -' -<V -' 




Insured not only hi.7;h electrical efficiency, but also greater mechanical 
strength tlian is c stomary. As a result of investigation of wind vel- 
ocity distribution at various heiri-hts above the earth. the antenna vm^ 
built to vrithstsnd an indicated wind velocity of 120 miles per hovr at 
the top and approximately 95 miles per hour at the bottom. jthe tovrer v,-as 
constructed of solid rour.ds electrically welded together into sections 
22 feet long, ^hese sections were fabricated in the steel mill and 
assembled by means of butt flanges Ln the field. As specified for all 
j^lBC to- ers, the complete ;':uy wire and insulator s; stem v;as assembled and 
stressed up to specified stress conditions in a si itably equipped test- 
ing plant. o-his eliminated any possibility of unsuspected weaknesses 
and al ^o Increases the modulus of elasticity of the f.vy cable. The 
use of track bonds around bolted io Int/^nsured satisfactory electrical 
connections. At the bottom is a horn gap for discharging lightning. 

h'^ ground system for this ne-zr structure consists of 66,0(0 
feet of copper ribbon buried in the si/ramp beneath it. The strop was '''-!/ 
buried ten Inches belov/ the surface ar.d was laid in position by means 
of a special plow which automatically raised a furrow, guided the ribbon 
into position beneath it, and partially covered it. The special plow 
hris been developed by KBC for this purpose. It is a modified "sub- 
soil er" wich was adapted for the purpose after experimentation with 
otlier types. A mounting was provided on v^hich a reel of copper ribbon 
could be quickly attached and the end of the ribbon guided to a roller 
at the rear of the --low : oint through w* ich it was deposited at the 
bottom of the furro^v. The special plov; or "sub-soiler" may be ad .-Justed 
for any depth of furrov>r desired. A tractor, preferably equipped with a 
bull-dozer, completes the equipment. Tne bull-dozer smooths up the 


f!;round after the work is completed, fills in brooks ar.d ditches by 
brute force, and moves logs, rocks, etc. Vdth an energetic creav, over 
6,000 feet of ribbon can be buried accurately and uniformly Ler day, 


The new 7ffiC transmitter is the HCA t; pe SD, which contftins 
facilities for chanp^ing power from one kilowatt for night broadea;' to 
""ive kilowatts for day operation by means of relcys. ihe transmitter 
is of the most recent design, and consists of seven units. There are 
four Ti^?in panel units, that is, the exciter unit, ti,e po-vYer-amclifier 
unit, the rodulator-rectif ier unit, and the control unit, and the three 
auxiliar\ units consistinf-, oi' the filter racks, the modulation trans- 
fonr'er-re'?.ctor assembly, and t?ie main plate tranformer. 

The transmitter is of vertical chassis design «nd contains 
high power tubes which dc not require water cooling apparatus snd 
radiators, formerly a necessary nuis:,. ce in Intermediate powered stations. 
These tubes are cooled by means of low speed fans -vhich make no per- 
ceptible sound. The absence of noise from blowers, contactors, and 
other tr nsmitter parts make possible high fidelity monitoring immediately 
adjaceiit to the transmitter. 

The transmitter features im; roved class B modulation in 

YV]||- o^^'^-i 

v^hich the idle plate current is less than 4C0 -siBMrs* The idle modulator 

plate current can be reduced to zero by a simple adjustment, with an 
increase of less than 1% in harmonic distortion. This system make'-- 
possible a very large increasein economy because t.^e power consumed by '' 
the modulator is confined to filair^ent lighting during periods of zero 
modulation. The total power consumed by the new transmitter is orly 
16 kilowatts during normal modulation, a reduction of abou.t 50^ from 







.01 ijrj',' TjK 

fi^al f'-huM>ii« 4'4iii><irMriitiii iiink<'« for 
aiaxuiiiini iti-i-rw^ibilili'. Hear \ ic^'W itC |iiiuc*r' 


older desio-ns of 5 kilowatt stations. 

The audio frequency system up to ami including the modulators 
is practically all resistance coupled to reduce phase shift. This makes 
possible an uiiusually simple and effective reversed feedback system 
vfhich reduces harmonic distortion and background noise tc extremely low 
values. One of the most attractive features about this feedback system 
is its utter simpliciby. There are no rectifier tubes, arjplifiers, or 
other parts to wear out or require replaceraer:t. As reference to the 
schematic circuit diagram, this feedback system operates from the 
primary of the modulation transformer to the secondary of the audio 
injut transfonner. The feedback s;. stem is a simple, permanently fixed 
resistive neti-rork. 

The radio frequency system utilizes duplicate low temperature 
coefficient quartz crystals with trimmers, bv which it is possible to 
quickly and easily adjust the transmitter to be exactly on its frequency'. 
The transmitting frequency is 950 kilocycles and never varies more than 
10 cycles. 

Adjacent to trsjismitter proper is the speech input equipment. 
The tvfo separate telephone lines running from the studio are terminated 
in this piece of equipment. The speech input equipment supplies the 
drop that occurs on the telephone lines and then passes signal to trans- 
mitter. The volume control unit is automatic and all circuits contain 
automatic circuit breeikers. The speech input equipment is made in 
duplicate and contains a means of locating trouble in case of signal 
failure. Counted with this equipment is a beat frequency meter and 
clocks to reR:ister the time the static is off the air during scheduled 


powe;r supply 

The station porter supply is provided in duplicate, incorporat- 
ing separate buried armored cables, transformer and metering ecuipment, 
terminating; ir; a fully a'^tomp-tic svfitch yj-h^eh instartly changes power 
lines in the event of a failure. The power lines are from Baltimore and 
Washington, D.C. The volta-e is 230 volts, three phase, 60 cycles 
vrith a po-'ver factor of approximately iQ%. The efficiency of tre trans- 
mitter is such that 7.15 kilo.vatt input is required for 5 kilo-att out- 
put and 1.43 kilowatt input is required for 1 kilowatt output. The 
automatic switch operates when any one of the three phases fails, and 
"'ill change from one line to the other so rapidly that the program is 
not interrupted, thus providirg better protection agaimt such failures 
than could be obtained by manual operation. 


WRC as equipped ;irith an }^CA type 69A distortion ji.eter ;3nd 
type d8A audio oscillator. vVith this combination oj' instruments tr.e 
percentage of distortion at any frequency within the transmitter rang;e 
may be m.easured quickly and accurately and the background noise level 
may "e measured with equal facil ty. To facilitate transmitter testing 
off the air the station was provided ivith a simple dummy antenna consirt- 
in': of Ohm s;,un resistor units connected to simulate t' e characteristic 
of the CO xial transmission line. The transmitter has a waterproof 
antenna-coa 'ial line coup>ling unit, together with the monitoring recti- 
fier, ihis monitoring rectifier provides t!ie ideal means of operating 
a remote readin^ antenna meter since it is in.;.erious to lir'htn'ns' 
discharp;es which would easily burn out a therm.ocou, le. jt also supplies 





the excitation for a relay which removes po',\rer in case cf an arc across 
the antenna tase following a lightning discharge, or in case carrier 
power is lost for any other reason. 

To be used in case of emergency is a small antenna approximately 
100 feet hicch, and containing all necessary equip; ent for operation. 
This antenna would reduce the loss in profit due to failure of a commer- 
cial "broadcast to a figure less tl- an flOO. This auxiliary antenna oper- 
ates on one fifth the wa' e. 

ihe transmission line which connects the radio transmitter 
■"rith the antenna is a two inch coaxial syste.i!, usin^ a new type of in- 
sulator Yvhich raised the breakdown voltage by 3007c. The line is buried 
a minimum of three feet in the earth. One of the final operations on the 
line consisted of closing small gas leaks caused by minute blow holes in 
the casting. Such leaks could not be detected by ar^ practical method 
except immersion in clean water, xhe line ma'ntained 40 pounds pressure, 
with no loss for three days, before it was finally placed into position 
and covered. At the end of the next five weeks there was still no notice- 
able loss so the pressure was reduced to 2 pounds. A section of this 
line, with the new insulators, vms tested before installation for voltage 
breakdoiym and heating. The heat run was made at a radio frequency volt- 
age equivalent to that which would be obtained with a transmitter -ower 
of 2,000 kilo-i-atts. In final position the line rests on flanks and is 
protected by split tile. 


The field intensity survey of the new YffiC station was made as 
soon as it -".ras placed in operation, xhe measurements s?iowed th- 1 the 
results expectd were slightly exceeded, and that "'.'RC excels in providing 



The RCA Metal Air-Cooled 

Tubes are standard, long-life tubes 

provided with a copper fin motinl- 




( ) 


C. L. Beach 

Engineering Seles 

RCA I/axLuf acturin^ Company 

J. G. Rogers 

Transmitter Engineer •;TRC 

RCA Revievf 

April 1938, Yol. II, number 4 
January 1939, Yol. Ill, nui..ber 5 
April 1959, Yol. Ill, number 4 

A. T. E. Journal